The post from Sean Erp says it all, good to have in mind when you try to explain to customers!
Source: https://blogs.technet.microsoft.com/ptsblog/2013/06/20/office-365-mail-flow-troubleshooting/
The post from Sean Erp says it all, good to have in mind when you try to explain to customers!
Source: https://blogs.technet.microsoft.com/ptsblog/2013/06/20/office-365-mail-flow-troubleshooting/